PSI - Issue 2_A

Daiki Nakanishi et al. / Procedia Structural Integrity 2 (2016) 493–500 Nakanishi et al/ Structural Integrity Procedia 00 (2016) 000–000

496

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Table 3 Result of Charpy impact test in 3% silicon steel

Surface transition temperature, vTrs (K)

Energy transition temperature, vTrE (K)

Heat Treatment(K)

Test Temperature(K)

Absorbed energy, vE (J)

Brittle area, BA (%)

Mark

293 353 423 473 293 353 423 473

4 6

100

99 93

1,073 normalizing

2A1

425

439

24

396

0

4 4

100 100

1,573 normalizing

2A2

424

429

141 371

68

0

Fig. 3 shows the fracture surface observation of Charpy impact test carried out at 293 K by SEM. Fig. 3 (a) and (b) shows that of 2A1 and 2A2, respectively. In 3% silicon steel, as it has been pointed out [Tsujii et al(2012)], the periodic traces of organizational boundaries whose interval is 20 – 100 μ m, considered to twin deformation that is not observed in the cleavage fracture surface of usual low-carbon steel, were observed. In particular, twin deformation is observed in the whole fracture surface and the unevenness is quite large in the fracture surface of 2A2. However, the absorbed energy is not high. By contrast, in the fracture surface of 2A1, twin deformation is partially observed but river pattern is formed on most of cleavage fracture surfaces in the same way as usual low carbon steel. Namely, it is assumed that twin deformation is promoted by coarsening crystal grain size. In relation to twin deformation, it is now well known that increasing grain size in metals and alloys decreases the twinning threshold stress and increases the tendency for twin deformation [Murr et al(2004)]. Also, quantitative crystal grain size for preventing twin deformation from forming plane-wave shock loading was revealed by the literature [Murr et al(2004)]. Generally the threshold stress of twin is lower than that of slip at low temperature and the same effect is recognized in the case of increasing grain size. In 3% silicon steel of 2A2, the block patterns of twin did not disappear at the maximum temperature that indicated the brittle fracture. On the other hand, the proportion of the block pattern was increased in the low temperature area of 2A1.

Fig. 2 (a) Observation of fracture surface of 2A1 in Charpy impact test

Fig. 3 (b) Observation of fracture surface of 2A2 in Charpy impact test